In the design of RF and microwave filters, it may be necessary to provide coupling between resonators. This is true for both bandpass and bandstop filter networks. The principles of network synthesis are used to determine the required values for the coupling coefficients, normally considered as frequency invariant numerical ratios, relative to a unit input level.
Because the coupling elements normally consist of either inductive or capacitive elements, or combinations of inductive, capacitive and resistive elements, the actual measured values of coupling are indeed frequency dependent. This difference between the assumed frequency independence and the actual frequency dependence causes the response of the fabricated filter to differ from the theoretical design.
It is possible to account for the frequency dependence of the coupling elements, within the original design. This tends to reduce the difference between the initial design and the actual fabricated filter. However, the degree of control required for the values of the coupling elements is so great as to significantly increase the fabrication cost. Tolerances have to be extremely tight and this is not normally practical.
To alleviate this problem it is desirable to have coupling mechanisms that are capable of adjustment during the filter tuning process. One method for achieving a significant range for realizing tunable couplings is taught by Snyder (U.S. Pat. No. 5,220,300). This method employs short evanescent waveguide sections, resonated as appropriate, to achieve either positive (inductive) or negative (capacitive) phase shift and, thus, realizes appropriately either inductive or capacitive coupling. The method requires that the short evanescent waveguide section be resonated (using a capacitance disposed across the short section) above the filter center frequency for achieving inductive coupling, and below the filter center frequency for achieving capacitive coupling. The length of the short evanescent section interacts with the resonating capacitance. The shorter the length, the larger the required capacitor. Small values of coupling use longer coupling lengths. Large values of coupling require short coupling lengths. Large capacitors tend to have low values of Q, and, thus, contribute to high filter insertion losses. Given that the mechanical design sometimes necessitates very short lengths in the coupling regions, it is desirable to have other methods for tuning the couplings to achieve large values of coupling, without the use of large resonating capacitors, or to achieve small values of coupling in a short length. Tunable couplings should also be capable of achieving either inductive or capacitive couplings, as required in the initial synthesis.
A conventional method for achieving capacitive coupling uses a capacitive probe. This is essentially an insulated section, typically supported by a dielectric sleeve within an iris opening, with the probe having proximity to both capacitively coupled resonating rods. The probe is long enough to reach from one resonating rod to the other, with a small capacitive gap between the end of the probe and the resonating rod. The capacitance is thus fixed, determined by the spacing between the end of the probe and the resonating rod. A conventional method for providing inductive coupling between resonating rods employs a similar probe, but with the probe ends terminated in a grounded loop of wire. Current flows in the loop to ground, and the concomitant magnetic field provides inductive coupling from the probe to the resonating rod, and thus from one resonating rod to the other. Again, the value for the inductive coupling is essentially fixed by the length and gauge of the grounded loop, and the proximity of the loop to the given resonating rod.
A conventional capacitive probe, generally designated as 10, is shown in FIG. 1. As shown, resonating rod 12 is separated from resonating rod 16 by septum 19. End 14a of resonating rod 12 is grounded and other end 14b is open circuited. Similarly, end 18a of resonating rod 16 is grounded and other end 18b is open circuited.
Septum 19 includes an iris opening (not labeled) for supporting probe 13, the probe being insulated from septum 19 by dielectric sleeve 15 made of Teflon. As shown, the probe is long enough to reach from resonating rod 12 to resonating rod 16 with small capacitive gaps between the ends of the probe and the resonating rods. The capacitance is thus fixed, and is determined by the spacing between the ends of the probe and the resonating rods.
An equivalent circuit of capacitive probe 10 is shown in FIG. 2 and is generally designated as 20. As shown, the equivalent circuit includes capacitor 22, transmission line 24 and capacitor 26 connected in series.
Referring to FIG. 3, there is shown a conventional inductive probe, generally designated as 30. As shown, resonating rods 32 and 36 are separated by septum 39. End 34a of resonating rod 32 is grounded and other end 34b of resonating rod 32 is open circuited. Similarly, resonating rod 36 includes end 38a which is grounded, and other end 38b which is open circuited. As shown, probe 33 is inserted in an iris of septum 39 and is insulated from the septum by dielectric sleeve 35, the latter being sandwiched between the septum and the probe. Each end of probe 33 is grounded by a grounded loop of wire. Current flows in the loop to ground, which generates a magnetic field and provides inductive coupling from the probe to each of resonating rod 32 and resonating rod 36.
An equivalent circuit 40 of inductive probe 30 is shown in FIG. 4. As shown, coil 42, with one end grounded, is connected in series with transmission line 44 and coil 46. Similar to coil 42, coil 46 has one end grounded and another end coupled to transmission line 44.
The transmission line forming the body of each probe, whether capacitive or inductive, cannot be avoided. It is essentially a coaxial line, including a probe and a dielectric sleeve for the probe, which is inserted in an opening of a wall (septum) separating two resonating rods. The transmission line length and impedance value result in a frequency variation in coupling to each resonating rod, causing even further deviation from a desired coupling coefficient. Accordingly, a tunable coupling, or an adjustable coupling between the probe and the resonating rods is very desirable. The present invention addresses such adjustable tunable couplers.